1. Field of the Invention
The present invention relates to transmission and reception of the ultrasound performed by using a capacitive micromachined ultrasonic transducer (cMUT) that is manufactured by using the micromachine process.
2. Description of the Related Art
The diagnostic ultrasound method has been attracting interest, in which the ultrasound is transmitted in a body cavity, the echo signals of the transmitted ultrasound is used to visualize the state of the body cavity in order to use the image for the making a diagnosis. One of the instruments that are used for this diagnostic ultrasound method is an ultrasonic endoscope. The ultrasonic endoscope has an ultrasonic transducer at the distal end of the insertion tube that is to be inserted into body cavities. This ultrasonic transducer can convert electric signals into ultrasonic beams in order to transmit the beams in the body cavities, and can receive the ultrasonic beams reflected in the body cavities in order to convert the received ultrasonic beams into electric signals. By performing the imaging process on the basis of these electric signals, ultrasonic images can be obtained.
In recent years, capacitive micromachined ultrasonic transducers (cMUT) that are obtained by silicon semiconductor processing by using the MEMS (Micro Electro-Mechanical System) technique have been attracting interest.
The cMUT is a device that has a bottom electrode arranged on a silicon substrate and an upper electrode on a membrane spaced from the bottom electrode by a cavity. When an RE signal is transmitted to one of the electrodes of the cMUT, a membrane including the upper electrode vibrates and generates the ultrasound.
It is said that the direct-current (DC) bias voltage is usually required to perform the transmission and reception of the ultrasound that uses the cMUT (as disclosed in International Publication Wo 2003/011749). The reason why the DC bias voltage is used for the transmission and reception of the ultrasound by using the cMUT is described below.
The object of using the DC bias voltage for transmitting the ultrasound is to cause the membrane to express the change of the waveform that is the same as that of the driving signal. When a high-frequency pulse signal (RF signal) is applied to the cMUT as a driving signal, the two electrodes only attract each other and will not repel each other because they are respectively charged with the electric charges that have opposite polarities. Accordingly, even when a signal having the sine wave is input as the RF signal, the membrane of the cMUT vibrates and deforms only in such a manner that the distance between the electrodes becomes shorter. Accordingly, the output vibration waveform does not correspond to the waveform of the input RF signal, and is distorted. Also, because the membrane vibrates and deforms only in such a manner that the distance between the electrodes becomes shorter, the amplitude becomes ½ with respect to the waveform of the input RF signal. Also, the frequency of the transmitted waveform becomes twice. Then, by applying a prescribed DC bias voltage to the input RF signal (sine wave), the distortion of the amplitude of the sine wave can be suppressed. Thereby, the transmission waveform from the cMUT can be caused to correspond to the distortion of the same waveform as the RF signal.
However, voltages that are relatively high (for example, 100V) are continuously applied as the DC bias voltage and the RF signal, accordingly the operation effective voltage becomes high. Also, when a type of a cMUT that is inserted into body cavities is considered, the outer dimensions are limited, which is different from a type of a cMUT that is used out of the body, thus has to be small. Accordingly, the applicants of this application disclosed a technique of operating a cMUT by applying a driving pulse signal obtained by superposing a DC pulse signal on an RF signal (International Publication: WO 2005/120359).
The object of using the DC bias voltage on receiving the ultrasound is to detect ΔV that is based on ΔV=Q/ΔC where ΔC is change of the capacitance occurring when the membrane of the cHUT (upper electrode) receives ultrasound with the upper and bottom electrodes being charged. By detecting this change of the charge, the received ultrasound can be converted into electric signals. As described above, in the conventional configuration, a constant charge Q (=CV, C:Capacitance without receiving ultrasound, V:DC bias voltage) that was accumulated in an electrode constituting the cMUT by using the DC bias voltage and the voltage ΔV=Q/ΔC (calculation based on the capacitance change ΔC that is accompanied with the change of the distance between the electrodes caused by the reception of the ultrasound) was detected by using a charge amplifier. Then, the impedance conversion and the voltage amplification are achieved. And the voltage output signal corresponds to the receiving ultrasonic signal, and diagnostic ultrasound images are obtained after the signal processing and image processing.
Conventional documents that relate to the present invention include Japanese Patent Application Publication Nos. 2003-294527, 2005-265432, and 2001-339796.